The processing of waste including municipal waste, medical waste, toxic and radioactive waste by means of plasma-torch based waste processing plants is well known. Referring to FIG. 1, a typical prior art plasma-based processing plant (1) comprises a processing chamber (10) typically in the form of a vertical shaft, in which typically solid, and also mixed (i.e., generally, solid plus liquid and/or semiliquid), waste (20) is introduced at the upper end thereof via a waste inlet means comprising an air lock arrangement (30). One or a plurality of plasma torches (40) at the lower end of the chamber (10) heats the column (35) of waste in the chamber (10), converting the waste into gases that are channeled off via outlet (50), and a liquid material (38) (typically molten metals and/or slag) which is periodically or continuously collected at the lower end of the chamber (10) via reservoir (60). Oxidising gases or fluids, such as air, oxygen or steam (70) may be provided at the lower end of the chamber (10) to convert char residues comprising carbon, produced in the processing of organic waste, into useful product gases such as CO and H2, for example. A similar arrangement for dealing with solid waste is described in U.S. Pat. No. 5,143,000, the contents of which are incorporated herein by reference thereto.
At least two problems are commonly encountered relating to the provision of oxidising fluids to the chamber that prevent smooth operation of such processing plants or furnaces. As waste is processed and proceeds to the lower, hotter parts of the chamber, inorganic waste in the form of molten or semi-molten material (including metal, oxides, salts and so on) may be deposited on the chamber walls, at times obstructing or at least partially obstructing the oxidising gas or fluid outlets into the chamber, as well as the plasma torch outlets. Once this happens, the deposited material can only be removed manually, i.e., by shutting down the plant, and after it cools down mechanically or otherwise removing the obstruction, or, by increasing the temperature in the chamber so that the deposited material (metal, oxides, salts and so on) melts and flows away from the oxidising gas outlet. The first solution results in plant down-time, with the ensuing economic penalties. The second solution is not always feasible, and requires more power to be provided to the chamber, which reduces the thermal and economic efficiency of the plant.
The second problem is in providing oxidising fluid or gases to the waste thoroughly and rapidly, so that all of the char in the waste may be converted into useful gases effectively and efficiently. In many prior art processing plants, oxidising gases are directed towards the axis of the chamber, and while penetrating the waste column to some extent, homogenous distribution is not achieved. In U.S. Pat. No. 5,143,000, steam is introduced tangentially into a lower part of the processing chamber, about half-way between the longitudinal axis of the chamber and the wall thereof. This configuration is also not very effective in distributing the oxidising fluid, since the amorphous and dense composition of the waste at this part of the chamber prevents the oxidising fluid from reaching all parts of the waste effectively. SU 1715107 describes a waste processing plant having a lower double-walled water cooled metal crucible, in which an inductor is used for providing heat to the waste, and an upper part made from refractory material, and having the same internal diameter as the lower crucible. Oxidising gas is provided at a location between the upper part and the crucible via a plurality of circumferentially placed openings therein, serviced by an external annular air pipe arrangement made from metal. This arrangement is specifically described with respect to a plant based on induction heating, and would not be suitable for a plasma-based plant, where the operating temperatures are much higher and refractory materials are usually necessary for the crucible part thereof, since the metal pipe would melt. In fact, the annular pipe configuration, while providing a multitude of oxidising fluid entry ports into the chamber, is not readily adaptable to processing plants in which the lower part of the processing chamber is made from a refractory material rather than metal, as such refractory material does not easily lend itself to having a plurality of bores drilled therein, since, for example, the mechanical strength of the furnace wall is substantially weakened. Further, providing a plurality of individual and separate locations along the circumference for injecting the oxidising gases is not fully effective in enabling the gases to penetrate into the waste column, in particular discrete inlets tend to get blocked during operation of the chamber, as mentioned hereinbefore, and the water cooling provided to the crucible results in some solidification and material deposition thereon, which exacerbates the blocking of the inlets.
Thus, none of the above patents, the contents of which are incorporated herein by reference thereto, provide adequate distribution of oxidising gases to the waste column, particularly in a plant in which the lower hotter part of the processing chamber is made from refractory material. Further, none of these patents disclose or suggest how to avoid obstruction of the oxidising gas inlet ports or plasma torches due to deposition of melted or semi-molten inorganic waste thereon.
It is therefore an aim of the present invention to provide an oxidising gas inlet system for enabling efficient and rapid introduction of oxidising gasses to the waste which overcomes the limitations of prior art plants.
It is another aim of the present invention to provide such a system that will allow the mixing of the relatively cold oxidising gases or fluid with the hot plasma gases generated by the plasma torches such as to enable the uniform and rapid reaction of preheated oxidising gases or fluid and char in the gasification zone.
It is another aim of the present invention to provide such a system that may be incorporated into a solid waste processing apparatus.
It is another aim of the present invention to provide a system for substantially avoiding the blocking of oxidising gas inlet ports and/or plasma torches in a plasma-torch type processing apparatus.
It is another aim of the present invention to provide such a device that is relatively simple mechanically and thus economic to incorporate into a processing chamber design.
It is another aim of the present invention to provide such a system incorporated as an integral part of a plasma-torch based type mixed waste converter.
It is also an aim of the present invention to provide such a system that is readily retrofittable with respect to at least some existing plasma-based waste converters.
The present invention achieves these and other aims by providing an oxidising fluid distribution channel or chamber incorporated into the design of the processing chamber, the channel having at least one oxidising fluid entry port or inlet associated therewith for introducing oxidising fluid therein from a suitable supply. The channel is recessed with respect to the waste column so that the oxidising fluids are initially separated from the waste column, enabling the oxidising fluids to be distributed circumferentially or at least peripherally around the waste column, thereby enabling the oxidising fluids to then penetrate into the waste column from all directions. Such a channel may be formed as an integral part of the chamber using refractory material. The plasma torches may be similarly located in a recess-type feature also incorporated in the refractory material, to minimise obstruction by deposited inorganic material.
While some plasma-based processing plants may have a recess or channel between part of the waste column and part of the wall of processing chamber, such recesses are not functional in the sense of the present invention. In other words, they are not directed to provide circumferential distribution of oxidising fluid to a part of the waste column in which char is being converted to product gases, and indeed are not adapted for so doing. Indeed, as exemplified by U.S. Pat. No. 4,831,944, some such prior art plants neither disclose nor suggest any facilities at all for providing oxidising fluid to the waste, much less for the circumferential distribution thereof, and are neither directed to solving the problems addressed by the present invention nor provide a similar solution. Japanese Patent Nos. JP 10110917 and JP 10089645 each describe a vertical melting furnace having a bulging mid-section in which are provided a plurality of combustion gas supply ports to form an annular combustion chamber. No plasma torches are used in these furnaces. Combustion gases are thereby provided to the annular combustion space provided in the mid-section in order to burn waste in the thermal decomposition zone and thus prevent or reduce bridging thereat. Air is provided to the furnace by means of a plurality of inlet ports at the lower end of the furnace, where the furnace cross-section returns to the original unbulged size. Such a system is not readily suitable for a plasma torch based processing plant. For example, some cooling of the melted inorganic materials at the lower end of the processing chamber would result due to the cooling effect of the air provided thereat, which could in turn cause blockage of the fluid inlet ports by solidifying inorganic material. Furthermore, while combustion gases are provided in the bulging section for burning product gases, there is no suggestion of supplying oxidising fluid thereat exclusively for the purpose of converting organic waste such as char into product gases. On the contrary, the aim of these patents is to reduce bridging and requires the addition of combustion gases within the bulging section to do so, oxidising air being provided to the furnace rather than the bulging section via the lower inlet ports, as described above. Such a system is thus not readily suitable for preheating oxidising gases and char inside the combustion chamber without the combustion of fuel therein.
U.S. Pat. No. 5,657,706 relates to an apparatus for processing waste, which is separated into three vertical sections. Waste is input via the middle section, the gasification chamber, and combustion air is delivered thereto via lateral openings level with the soleplates (not shown in the Figure). However, there appears to be no disclosure or suggestion of a distribution and mixing chamber for distributing and mixing oxidizing fluid around a column of waste. In particular, no mixing and distribution chamber is described nor hinted at having a peripheral opening in continuous peripheral fluid communication with a column of waste, nor having an outer peripheral wall which is formed as in the present invention.
EP 850,885 relates to a melt treatment apparatus which comprises a passage which is laterally displaced from the main chamber, and which comprises an inlet for a gas such as air. The passage is provided for melt discharging and is not for distributing oxidising fluid around any waste—the location of the gas inlet is substantially downstream of the lower end of the waste. No distribution and mixing chamber for oxidizing fluids is disclosed or hinted at, less so one having a peripheral opening in continuous peripheral fluid communication with a column of waste, nor having an outer peripheral wall which is formed as in the present invention.
EP 837,041 relates to a plasma treatment of ashes, in which a number of tiltable electrodes are provided in a plasma-based apparatus. While the wall of the lower part of the apparatus is laterally displaced with respect to an upper part thereof, the lances which are used to provide air and steam are located in the upper part of the apparatus, and are thus not directly associated with the displaced wall of the lower part. Furthermore, there is a lack of fluid communication between the upper part of the chamber comprising these lances, and the lower wider part of the chamber, due to a blocking plug of waste in the upper part that is present during operation of the chamber. There is no disclosure or hint of a mixing and distribution chamber for distributing oxidizing fluids from the lances around a lower part of the column of waste, less so as in the present invention.
EP 625,869 relates to a plasma arc apparatus that may be used for treatment of waste. A melter shell is provided having a smaller diameter than the outer melter hood, providing an annular gap to allow ingression of air into the plasma arc furnace. Thus, as illustrated in the figures, an inner facing wall of the upper part (including the hood) is actually outwardly displaced with respect to the lower part of the apparatus (the melter shell), which is the reverse of the arrangement of the present invention, as will become evident hereinbelow. Furthermore, the only oxidising inlet to the chamber is the upper central port, which lies in the middle of the upper part of the column of waste. Thus, in such a configuration, there is no fluid communication between the upper part of the chamber comprising this inlet, and the lower part of the chamber, due to a blocking plug of waste that is present therein during operation of the chamber. There is no disclosure or hint of a mixing and distribution chamber for distributing oxidizing fluids from the lances around a lower part of the column of waste, less so as in the present invention.
These references fail to disclose or suggest a distribution and mixing chamber having:                a peripheral opening in substantially continuous peripheral fluid communication with a lower part of a said column of waste when said column of waste is accommodated in said lower part and in fluid communication with at least one said oxidising fluid inlet;        an outer peripheral wall thereof formed by a lateral outward displacement of an inwardly-facing wall of said lower part of said processing chamber with respect to an inwardly-facing wall of said upper part of the processing chamber; and        wherein at least one said oxidising fluid inlet is separate from and associated with said at least one plasma torch means such that during operation of said apparatus oxidising fluid flowing from the said at least one oxidising fluid inlet into said distribution and mixing chamber is directed at a high temperature zone provided by the at least one plasma torch means that is associated with said at least one oxidising fluid inlet.        
Thus, not only are these references are not directed towards providing a mixing and distribution chamber as in the present invention, but they describe features of processing chambers which are different in structure and function to the mixing and distribution chamber of the present invention.
In general, plasma-based processing plants which only provide oxidizing agents via the plasma torches are also limited in that if more oxidising fluid is needed for processing the char, this results in a corresponding lowering of the temperature produced by the plasma torches. This in turn results in greater inorganic material deposits, which in turn cause the congestion problems discussed above. On the other hand, if it is desired to increase the temperature of the plasma torches, the oxidising fluid provided thereto must be reduced, which results in char in the waste column not being fully oxidised into product gases or alternatively the power that needs to be provided to the plasma torches has to be raised, which reduces the efficiency of the process. Thus, providing an oxidising fluid inlet, independent from the plasma torches, allows additional freedom in the way in which the processing plant may be used, as this enables both the temperature of the plasma jets and the volume flow rate of oxidising fluid to be increased simultaneously as required. However, plasma torch based furnaces which employ oxidising fluid inlets independent from the plasma torches are not necessarily without problems. Such furnaces need to be designed such a manner that the oxidising fluid inlets, which provide relatively cool oxidising gases or fluids, are sufficiently spaced from the inorganic melted products, and in particular from the discharging outlets thereof. Otherwise, congestion of these discharging outlets may occur due to the cooling of the melt (and solidification thereof) by the action of the relatively cool oxidising fluids on the melt.